1. Field of the Invention
The present invention relates to a battery pack charger capable of charging a battery pack while preventing the occurrence of overcharging. The term "battery pack" as used herein contains a plurality of battery cells connected in series and covered by an external frame typically formed of resin, and is provided with both charging and discharging terminals.
2. Description of the Prior Art
Technology for rechargeable batteries and rapid charging of such batteries has become more and more advanced in recent years, thereby greatly increasing the use of portable battery-powered equipment such as cordless electric tools. When rapidly charging batteries, it is essential that the charging operation is stopped when the battery has reached a full charge. If rapid charging is continued after the battery has reached a full charge, the battery may become extremely hot, causing damage to the battery or even the breakout of fire. The extremely high temperatures caused from such overcharging may also deform the external frame or damage the control circuit of the battery charger.
Battery chargers are provided with a charging control circuit for terminating the charging operations when the battery has become fully charged but there is always a possibility that this control circuit will malfunction. For this reason, it is necessary to provide an overcharge prevention mechanism that is separate from the control circuit. Conventionally, this overcharge prevention mechanism has been accomplished using a heat-sensitive switch within a battery pack.
FIG. 1 is a circuitry diagram showing the state when a conventional battery pack 10 is connected to a battery charger 1. The battery pack 10 includes a plurality of battery cells 11 connected in series; a charging terminal 13; a charging/discharging terminal 12; a discharging terminal 13a; and a heat-sensitive switch 9 connected between the charging terminal 13 and the positive polarity of the battery cells 11. The battery pack 10 has an outer appearance as shown in FIG. 3.
During charging operations, a terminal 8 of the battery charger 1 is connected to the charging terminal 13 of the battery pack 10, and a terminal 7 of the battery charger 1 is connected to the charging/discharging terminal 12 of the battery pack 10. The charging current flows in a path from the charging terminal 13, to the heat-sensitive switch 9, the plurality of battery cells 11, and the charging/discharging terminal 12.
The discharging terminal 13a is exclusively used during discharging operations. That is, a battery-powered equipment is used while connecting it to the discharging terminal 13a.
In a battery charging system having the above-described construction, if the charging control circuit of the battery charger is incapable of properly terminating the charging operation for any reason, the resulting abnormal heat emitted from the plurality of battery cells 11 will cause the heat-sensitive switch 9 to open, effectively stopping the charging operation. If the heat-sensitive switch 9 is designed to open at an operating temperature of approximately 80.degree. C., no substantial performance loss will be incurred to the battery cells 11.
The heat-sensitive switch 9 contained in the battery pack 10 serves as a fail-safe element so that this type of the battery pack 10 will never be overcharged. However, when charging a battery pack that does not contain a heat-sensitive switch 9, the occurrence of overcharge will not be prevented unless the battery charger 1 properly terminates the charging operation.
To overcome this problem, a battery charger 1 as shown in FIG. 2 has been proposed. As shown in the drawing, this battery charger 1 includes external frames 1a and 1b; a control circuit board 3 housed within the external frames 1a and 1b; terminals 7 and 8 (terminal 8 is not shown); and a heat-sensitive switch 9. The heat-sensitive switch 9 is mounted on the control circuit board 3 and inserted into an opening 15 provided in the external frame 1a so that heat emitted from the battery cells 11 is transferred to the switch 9 via the external frame 1a.
The battery pack 10 is provided with an external frame 14; battery cells 11 housed in the external frame 14; and terminals 12 and 13 (terminal 13 is not shown) fused to the positive and negative electrodes, respectively of the battery cells 11. The battery pack 10 is inserted into an insertion opening formed in the external frame 1a of the battery charger 1.
The heat-sensitive switch 9 prevents overcharge of the battery cells 11 by stopping the charging operation when the switch detects an abnormal amount of heat emitting from the battery cells 11. Heat from the battery cells 11 is transferred to the heat-sensitive switch 9 both via the battery pack terminals 12 and 13, the battery charger terminals 7 and 8, and the control circuit board 3, and via the external frame 14 of the battery pack 10 and the external frame 1a of the battery charger 1. When this transferred heat increases the temperature of the heat-sensitive switch 9 above a predetermined value, the heat-sensitive switch 9 opens to stop the charging operation.
However, the amount of heat transferred via the external frames 14 and 1a is extremely small, and the heat transferred via the control circuit board 3 is cooled by the same, causing the amount of heat actually transferred to the heat-sensitive switch 9 to be much smaller than the heat emitted from the battery cells 11. Hence, by the time the heat-sensitive switch 9 is activated, the battery cells 11 have reached an abnormal temperature that could potentially cause the battery cells 11 to overcharge, deform the external frames 14 and 1a, and damage the control circuit.